"We do research in an ongoing way, in a very privacy-protected way, to improve our service, and this was done with that goal. I think we are in communication with regulators all over the world and this will be OK. And we will continue to make sure users understand that we care about their privacy. We care about their experience, and we want to do everything we can to give them the best experience we can."

In January 2012, researchers from Facebook, Cornell University, and the University of California at San Francisco conducted a study called "Experimental Evidence of Massive-Scale Emotional Contagion Through Social Networks." During the course of the study, without publicly disclosing what it was up to, Facebook altered the News Feeds of 689,003 random users to show either more positive or more negative posts -- and thus possibly altered their emotional states, with the goal of finding out whether good vibes or bad would be contagious.

But it was just less than a week ago that word spread about the study, to considerable backlash.

"I can understand why some people have concerns about it, and my coauthors and I are very sorry for the way the paper described the research and any anxiety it caused," Kramer wrote on Sunday. " In hindsight, the research benefits of the paper may not have justified all of this anxiety."

This was hardly the first time Facebook has had to apologize for taking unexpected liberties with people's experience of the mammoth social network, which now counts 1.2 billion users around the world. In 2012, for instance, Facebook reached a settlement with the Federal Trade Commission to deal with charges that it had deceived users, telling them they could keep information on the network private, but then allowing it to be made public.

On Wednesday, Sandberg chalked up the recent episode to poor communication about a trifle.

"Facebook has apologized and certainly never wants to do anything that upsets users and particularly for communicating really badly in this case," Sandberg said.

Sandberg also downplayed the extent of the project, which she said lasted one week, calling it "a small experiment."

"It has been communicated as an experiment to shift emotions. it's not exactly what it was," she said. "It was an experiment in showing people different things to see how it worked. And again, what really matters here is that we take people's privacy incredibly seriously and we will continue to do that and give people control and a great experience."